I have an ARC SP-10 that seems to have a problem. After about 45-50min of playing, the signal suddenly cuts off. All sources. I am left with a faint hum, that disappears when I flip the mute/operate switch on the front panel. Only solution seems to be to turn it off, let it cool down, the power it up again. Then everything is fine....for 45-50min.

The circuit includes a turn on mute relay that delays full turn on for 2.5min. The power on LED flashes until the relay trips after 2.5min. This LED does not flash when the signal cuts off as outlined above.

Could the problem be a malfunction of the turn on relay or something else ?

It's not sophisticated preamp and could be repaired by local tech.You can also chat with Chris from partsconnexion about possible upgrade of circuit elements of your preamp to give it newer and stronger life.